1. Field of the Invention
The present invention relates to an active matrix substrate, a liquid crystal display device, and a method for fabricating such an active matrix substrate. More particularly, the present invention relates to an active matrix substrate and a liquid crystal display device which are thin, inexpensive, and excellent in display quality (i.e., free from color blur or color mixture and having a high aperture ratio to provide bright display images), and a method for easily fabricating such an active matrix substrate.
2. Description of the Related Art
In recent years, color liquid crystal display devices have been increasingly used for liquid crystal TV sets and various information apparatuses. For such color liquid crystal display devices, a color filter composed of colored pixels including color ink of the three primary colors, i.e., red (R), green (G), and blue (B) arranged on a transparent substrate is generally used.
Conventionally, such a color filter is typically fabricated by a photolithographic method where a photo-sensitive resist with pigments dispersed therein is patterned in a photolithographic process. According to this method, the photolithographic process is required to be repeated three times for the R, G, and B colored pixels. This complicates the fabrication process of the color filter, and thus increases the cost.
Such a color filter is generally disposed on a counter substrate of a liquid crystal display device, not on an active matrix substrate with thin film transistors (TFTs) formed thereon. This is because, if a color filter is disposed on an active matrix substrate, the following problems arise: (i) the reliability of TFTs may be lost due to the formation of the color filter; (ii) the production yield which has been lowered by the formation of the TFTs may be further lowered by the successive formation of the color filter; and (iii) it is difficult to form the color filter on the surface of the active matrix substrate having a complicated profile. When the color filter is disposed on the counter substrate, however, the following problem exists. That is, it is extremely difficult to precisely align pixels of the color filter with pixels of the active matrix substrate at the attachment of the counter substrate with the active matrix substrate in the fabrication of the liquid crystal display device. A margin is therefore required for an error of the alignment. For this purpose, a light-shading layer formed above pixel electrodes of the active matrix substrate is made somewhat larger. This reduces the entire area of the pixels, darkening resultant display images.
In order to overcome the above problem, Japanese Laid-Open Patent Publication No. 7-134290 proposes a liquid crystal display device using a color filter formed by directly applying color ink to pixel electrodes of an active matrix substrate. More specifically, R, G, and B color ink drops are applied to pixel electrodes of an active matrix substrate by an ink jet method to form a color filter directly on the active matrix substrate. This method does not require the photolithographic process and thus simplifies the fabrication process. As a result, an inexpensive color filter can be fabricated with high yield. Moreover, no consideration of an error in the alignment of pixels of the color filter with pixels of the active matrix substrate is required at the attachment of the counter substrate with the active matrix substrate in the fabrication of the liquid crystal display device. A light-shading layer is therefore not required to be enlarged for an alignment error, and thus the aperture ratio can be improved.
The above liquid crystal display device however has a drawback. In particular, uniform drops of color ink are injected from a nozzle by the ink jet method. Such color ink drops isotropically spread over pixel electrodes of a substantially rectangular shape. When the color ink is injected to spread to every corner of each pixel electrode, however, it intrudes beyond the pixel electrode into a portion which is not intended to be colored with this particular color ink, thereby causing color blur or color mixture between adjacent pixel electrodes.
In order to prevent such color blur and color mixture, Japanese Laid-Open Publication No. 4-123005 proposes a method for controlling the wettability (i.e., hydrophilicity/hydrophobicity) of the surface to which color ink is to be applied. In this method, however, a hydrophobic resin film for preventing color blur and color mixture must be formed in addition to a protection film for protecting wirings on the active matrix substrate. This causes many other drawbacks such as the increased thickness of the active matrix substrate (and thus the resultant liquid crystal display device), the complicated fabrication process, and the increased fabrication cost.
In view of the above problems, it is desired to provide an active matrix substrate and a liquid crystal display device which are thin, inexpensive, and excellent in display quality (i.e., free from color blur or color mixture and having a high aperture ratio to provide bright display images), and a method for easily fabricating such an active matrix substrate.